Before playing for Distillery in the Irish League, O'Neill played for the South Belfast side Rosario. Now he also has a conference room dedicated to him in Rosario Football Club's local Youth Club. While at Distillery, he won the Irish Cup in 1971, scoring twice in a 3-0 win over Derry City in the final.[6][7] His second goal was particularly impressive, a mazy run in which he dribbled past three opponents before scoring with a powerful shot.[8] As a result of winning the cup, Distillery qualified for Europe the following season. O'Neill scored against Barcelona in the European Cup Winners' Cup in a 3–1 home defeat in September 1971.[6][9] During this period he was spotted by a scout for Nottingham Forest. He signed for Nottingham Forest in October 1971, leaving Distillery for a transfer fee of £15,000 and quitting his studies in law at the Queen's University of Belfast.[10]

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O'Neill went on to play an integral role in Forest's golden era. He scored on his league debut for the club, a 4-1 win over West Bromwich Albion on 13 November 1971.[11] He went on to make a total of 17 league appearances that season, scoring twice, but couldn't prevent his side's relegation from the First Division in 1972.[11] However, the appointment of Brian Clough as manager in January 1975 was the beginning of a revolution for Nottingham Forest.[12] Under Clough's management, O'Neill helped Forest gain promotion to the top flight in 1977, won the league title and League Cup a year later,[13] followed by further League Cup success a year later.[14] He was dropped to the substitutes bench for Forest's first European Cup victory over Malmö in 1979 after failing to fully recover from an injury,[15][16] but he played in their 1980 win over Hamburg.[17]

At club level he also played for Norwich City, Manchester City and Notts County. O'Neill attempted to make a comeback in 1984 with Chesterfield, but only played part of a reserve game before being forced off with a knee injury after 20 minutes. This was made in an attempt to get fit for Northern Ireland's 1986 World Cup squad. After leaving Chesterfield, then-Fulham manager Ray Harford invited O'Neill to join the club in a bid to regain his fitness. O'Neill only managed to take part in two reserve games for Fulham (neither of which he completed), before retiring as a result of his knee injury in February 1985.[18][19]

He became manager of Wycombe Wanderers in February 1990. He played in the Martin O'Neill XI side, along with George Best, in the last match to be played at Loakes Park. In the 1990–91 season, he took Wycombe to fifth in the Football Conference. In the 1991–92 season, he led Wycombe to 2nd place in the Conference, losing out to Colchester United only on goal difference. The following season, he took Wycombe into the Football League for the very first time. In the 1993–94 season, he took Wycombe to a second successive promotion via the Division 3 play-offs and a 4–2 win over Preston North End took them up into Division 2.[23] In the 1994–95 season, Wycombe narrowly missed out on the Division 2 play-offs and he left the club on 13 June 1995 to become manager at Norwich City. O'Neill also won the FA Trophy with Wycombe in 1991 and 1993.[24]

As of December 2012, he remains as Wycombe's most successful manager in their history.

O'Neill became manager of Norwich City in June 1995,[25] and left the club in December, due to differences with club chairman Robert Chase over the potential signing of striker Dean Windass, during his first stint at Hull City for £750,000.[26]

In October 1998, he was favourite to take over the manager's job at Leeds United.[31]George Graham, who had just resigned from Leeds, brought his Spurs team to Filbert Street for his first game in charge. Leicester Mercury organised a protest and printed thousands of “Don’t Go Martin” posters, which were held up by fans throughout the game, which Leicester won. Thousands of balloons were also released. O’Neill remained as Leicester manager until his contract expired.

O'Neill left Leicester on 1 June 2000,[32] taking over from the team of John Barnes and Kenny Dalglish to become manager of Celtic, who had finished runners-up to Old Firm rivals Rangers in both of their previous seasons; in the season just gone, they had finished 21 points behind the champions.

O'Neill's first Old Firm game, in late August 2000, ended in a 6–2 victory for Celtic over Rangers.[33] It was their biggest victory over Rangers since the 1957 Scottish League Cup Final. His second Old Firm game saw a reversal of fortunes, however, as Celtic suffered a 5–1 defeat.[33] In that first season, O'Neill won a domestic treble with Celtic, the first time this had been achieved since 1968. He was then touted as a potential successor to Alex Ferguson, who had announced he was to leave Manchester United in 2002.[34] Celtic then retained the league title in 2001–02, the first time since 1982 that Celtic had managed that feat. Celtic also qualified for the Champions League group stage, winning all of their home games but losing all of their away games.

He then guided Celtic to the 2003 UEFA Cup Final in Seville, which Celtic lost 3–2 in extra time to a Porto side managed by José Mourinho.[35] This was Celtic's first European final since 1970 and they beat Blackburn, Celta Vigo, Stuttgart, Liverpool and Boavista on the way to the final. 80,000 Celtic fans travelled to Seville. The following season Celtic regained the league title from rivals Rangers and reached the quarter finals of the Uefa Cup, including a victory against Barcelona.

On 25 May 2005, Celtic announced that O'Neill would resign as manager to care for his wife Geraldine, who had lymphoma. His last competitive game in charge of Celtic was the Scottish Cup final 1–0 victory over Dundee United on 28 May 2005, decided by an eleventh-minute goal by Alan Thompson.[36]

Under O'Neill, Celtic won 213, drew 29 and lost 40, of 282 games played, and he was the most successful Celtic manager since Jock Stein. In his five seasons at Celtic Park, O'Neill won three League titles, three Scottish Cups, and a League Cup. The two league titles he lost were by margins of a point and a goal. He also oversaw a record 7 consecutive victories in Old Firm derbies, and in season 2003–04 Celtic created a British record of 25 consecutive league victories.[37]

O'Neill was introduced as the Aston Villa manager at a press conference on 4 August 2006. At the press conference he stated "It's absolutely fantastic to be back and with a club such as this. This is a fantastic challenge. I am well aware of the history of this football club. Trying to restore it to its days of former glory seems a long way away – but why not try? It is nearly 25 years since they won the European Cup but that is the dream."[38]

Villa had the year's longest unbeaten start of any Premier League side in 2006–07 (9 games), not losing a league game until 28 October. Villa suffered a mid-season slump but recovered late in the season, winning their three away games in April, to end the season how it began with a run of 9 unbeaten fixtures. For this O'Neill scooped the Barclays Manager of the Month for April.[39] Villa's final points tally was 50,[40] an improvement of 8 over the previous season and finished 11th, 5 places higher than the previous season.[41] In October 2007, Aston Villa owner Randy Lerner said that he would not stop O'Neill from leaving Villa if he was offered the vacant post of England manager.[42] O'Neill later dismissed the reports, calling them "unfair speculation".[43]

Aston Villa just missed out on a UEFA Cup spot on the final day of the 2007–08 season and qualified for the Intertoto Cup by finishing 6th. They scored 71 goals, (their best ever tally in the Premier League and best tally since winning the title in 1981), gained 60 points which was Villa's highest points tally since 1996–97, and were the third highest goalscorers.[44] After 25 games of the 2008–09 season, having qualified for the UEFA Cup as joint winners of the Intertoto Cup, the club were third in the table on 51 points, 2 points above Chelsea on level games and 7 points above Arsenal in 5th place and on course for a place in the Champions League for the first time since 1983. O'Neill decided to prioritise Champions League qualification above all else, fielding a virtual reserve side for a UEFA Cup game against CSKA Moscow which was subsequently lost.[45] Following this, Villa failed to win any of the next 10 league games and improving form for Arsenal & Chelsea meant that Villa failed to reach the top 4.

At the start of the 2009–10 season Villa failed to qualify for the group stage of the newly named Europa League, but continued their progress in the league with wins against Manchester United, Chelsea and Liverpool.[46] Arsenal defeated Villa 3–0 at Emirates Stadium, and drew at home.[46]

Once again Villa finished 6th for the 3rd season running, and once again improved their points tally finishing with 64 points; their poor home form (they drew 8 times at home) denied them a chance to qualify for the UEFA Champions League.

Aston Villa reached their first final under Martin O'Neill, and first final in 10 years on 28 February 2010 against Manchester United in the League Cup, but lost 2–1.[47]

On 9 August 2010, O'Neill resigned as manager of Aston Villa with immediate effect.[48] On his departure O'Neill said "I have enjoyed my time at Aston Villa immensely. It's obviously a wrench to be leaving such a magnificent club."[49] O'Neill was reportedly unhappy about the funds available for transfers,[50] but his departure just five days before the start of the new season still came as a shock to the club and its players.[50] Lerner issued a statement two days later saying he and O'Neill "no longer shared a common view as to how to move forward, but the two remain good friends."[51]

On 3 December 2011, O'Neill signed a three-year contract with the Premier League club Sunderland, the team he had supported as a boy.[52] In O'Neill's first game in charge Sunderland came from 1–0 down to beat Blackburn Rovers 2–1 at the Stadium of Light. Under O'Neill, Sunderland began to improve dramatically with four wins from his first six games, including one over league leaders Manchester City. The Daily Telegraph commented that Sunderland could make a late challenge for a European place if they kept their performances up.[53] Sunderland continued to perform well in the first few months under O'Neill. They rose to ninth in the league and continued their challenge for a Europa League spot. On 18 February, they beat Arsenal 2–0 to knock them out of the FA Cup fifth round. A week after this they lost 4–0 to West Bromwich Albion.[54] The next week was O'Neill's first Tyne–Wear derby. The 'fiercely contested' match finished 1–1 with two red cards for Sunderland.[55] The following week Sunderland defeated Liverpool 1–0 at the Stadium Of Light. Sunderland's form petered out at the end of the season and after no wins in the last 8 games but they finished a respectable 13th place, a position Sunderland fans would've been happy with after the start to the season.

The following season, O'Neill had bought Steven Fletcher and Adam Johnson in an attempt to build on his previous 13th place and push on for the top 10. He claimed a solid 0-0 draw at the Emirates against Arsenal in the first game of the season. Sunderland went unbeaten for the first five games before a 3-0 defeat at Manchester City. They then claimed a 1-1 draw in O'Neill's first Tyne-Wear derby at the Stadium of Light thanks to a late Demba Ba own goal. Sunderland then suffered a surprising 0-1 home defeat to Aston Villa and a 1-0 defeat to Middlesbrough in the League Cup. After a 2-4 home defeat to West Brom, rumours circulated that O'Neill had resigned. These were all quashed quickly and O'Neill continued despite slipping into the relegation zone following a 1-3 home defeat to Chelsea. Sunderland's form started to improve over the winter as they climbed the table following an impressive run of results including another 1-0 success over Manchester City and a 2-3 success over Wigan as they reached a season high of 11th. However, this proved to be O'Neill's last victory as Sunderland endured a run of 8 games without a win.

Martin O'Neill was sacked by Sunderland on 30 March 2013 following a 1-0 defeat by Manchester United which left the team one point above the Premier League relegation zone with seven games left to play in the season. Sunderland had failed to secure victory in the eight matches leading up to O'Neill's departure, winning only three points out of a possible 24 during that spell.[56][57][58]

O'Neill and his wife Geraldine have two daughters.[70][71] When he was a boy he supported Sunderland and Celtic. His favourite player was Sunderland's captain and centre half Charlie Hurley, who eventually won Sunderland's Man of the Century award in 1979.[72][73]

O'Neill courted controversy in March 2016 following comments he made about the physical appearance of players female partners, which were condemned as sexist. According to Orla O'Connor of the National Women's Council of Ireland: "There is no place for sexist comments of this nature in Irish football, particularly at this level" [77]

In June 2016, O'Neill was strongly criticised for using a derogatory term to describe LGBT people. During a public appearance in Cork that month, O'Neill informed a gathering of Ireland football fans that he had two others accompany assistant Roy Keane and him on a recent visit to San Francisco as he was worried people might think they were "queers". The Gay and Lesbian Equality Network (GLEN) condemned the offensive comments and called on him to apologise to the LGBT community. The National LGBT Federation also called on O'Neill to withdraw the remarks and apologise, noting that O'Neill's behaviour could only have a detrimental effect on attempts to tackle homophobic prejudice in sport. He eventually apologised for the derogatory remark a number of days later.[78][79][80][81][82]